The head of the company that built the Lamma IV ferry involved in the fatal National Day collision told the inquiry into the disaster its seats were not designed to withstand a collision.

The seats were secured firmly to its deck, Ken Lo Ngok-yang said.

Rescuers and survivors of the vessel's collision with the Sea Smooth off Lamma Island had previously told the commission that most of the upper deck seats became detached after the crash, trapping passengers as the ship sank. Some of those trapped eventually died. The commission-appointed naval architect, Dr Anthony Armstrong, mentioned in his report that the seats were loosely secured.

But Lo, the director of Cheoy Lee Shipyards, which built both the Lamma IV and Sea Smooth, vigorously defended his company yesterday, insisting that the seats were adequately secured and still useable even after 16 years. "If you rock your seat all the time, the seat … will crack or fall apart," he said. "For this type of vessel, I don't think anybody would have assumed that [seats would withstand collision]. This is not a requirement."

The veteran shipbuilder said seats were usually secured by screws to a fibreglass deck with bedding compounds to prevent water from seeping in and causing the screws to rust. Lo also addressed Armstrong's report that a missing watertight door had led to the flooding of three compartments and contributed to the speedy sinking of the ship. "It was never meant to have a door on it from day one," he said.

But counsel for the commission Roger Beresford pointed out that on the drawings of the ship, a "W.T. Door" marked the access opening of the aft bulkhead, suggesting that a watertight door had been included in the plan.

In response, Lo said Cheoy Lee had instructed an outsourced designer to design the Lamma IV based on another vessel similar to what Hongkong Electric was looking for, and he believed the designer had made a mistake and did not remove the word "W.T." from the drawings.

But he admitted that the error was discovered only after the accident.

Marine Department ship inspector Louk Hon-ying said measurements made in 2005 of the Lamma IV's hull plating thickness were mostly 4.5mm rather than the 5mm stated in the drawings. Armstrong's report had suggested that the thinner plating might have contributed to the extent of damage to the hull, but Louk said the difference fell within an acceptable range of error.

Calculations made by an independent naval architect appointed by the Department of Justice showed that the Lamma IV would sink when its last three compartments were flooded and with an 8.25-tonne ballast installed in 1998 at the ship's rear.